I've built a few XM-L based lights now... a couple Mag conversions and several single 18650 Li-Ion based units as well. All were built for duty use - so I had been potting drivers and leds with Arctic Alumina epoxy - and it works well except the cost -- for my mini production needs AA is expensive on a per flashlight basis. So, after some research I believe I have come up with a simple resin-based clear potting epoxy and hexagonal boron nitride powder combination that will not only outperform Arctic Alumina (AA is actually mostly aluminum oxide which has roughly half the thermal conductivity of boron nitride) but should be much cheaper. But here's the twist - I have to purchase fairly large quanitites. So what to do with a couple of GALLONS of BN epoxy? Well - if enough people are interested I could offer large syringes of the mixed A-B BN epoxy (say 50ml total for the price of AA and shipping - much larger) here for forum members.
I'm not trying to "sell" a product here on BLF - rather I'd like to make my costs for the flashlights I build a little more reasonable and see if this would help any BLF forum members as well... Your thoughts and comments are appreciated...
PS If you needed the "ultimate" in high-performance epoxy you could use powderized diamond - 5 times the thermal conductivity of copper... but of course at about $50-$60 for 100 GRAMS... it would defeat the cost-effective goal - but hey just FYI and for that 1-off high power flashlight it might be fun too.
You're right - alumina is very cheap and would make a great "budget" additive, but I still wanted higher performance so I can fill the entire head of a mag or other high-output for more thermal mass and great transfer properties without the extreme cost of diamond powder. Boron Nitride has an interesting twist as well - it's very hygroscopic so when I make "batches" of epoxy I will use 1kg sealed bags entirely and immediately. I have a dehumidified and temp controlled room I can use for this and I plan the use a single-stage vacuum pump and separate vessels for part A and B epoxy components. According to the resin manufacturer I can use up to a 1 to 1 ratio by volume of filler, but to maintain the epoxy's integrity I probably won't go above a 0.85 to 1 mix. But I will test at least a few combinations for their thermal properties. I will also test my mix versus a known "standard" like Arctic Alumina. To test the thermal transfer properties I will use my controlled environment and a peltier-heatsink assembly with a regulated power supply. If there's enough interest, I hope to begin in a month or two. I have most of the equipment - but need to order materials. The resins are sourced locally, but I will be using an overseas manufacturer for the BN - Asian manufacturers charge 30-50% less per kg.
I hope that answers some of your concerns - and if/when I can proceed with small batch production I will post more info and pictures.
Why use boron nitride? Silicon carbide has four times the thermal conductivity, is not hygroscopic, and is dirt cheap. Search Ebay for silicon carbide powder and you will find 1500 grit (very fine powder) for between $10 and $1 a pound.
Silicon carbide is an excellent electrical conductor which is used in electrical components such as resistors and the like. I use this epoxy for potting drivers and emitters so an electrical conductor is really not wanted. So far, the best non-conductive and non-capacitive compound I have found other than powdered diamond is hexagonal boron nitride. Thanks for the suggestion though.... Nutz
Nope, raw silicon carbide is an excellent insulator! It only becomes a conductor when doped with precise levels of impurities (just like silicon). I just stuck my ohm meter leads into a bag of it (the 1500 grit stuff off of Ebay). No reading at all... and the meter that I used goes to 2000 gigohms!
Maybe I'll pick some up and see what happens when it's encapsulated in epoxy. No manufacturer (that I know of) is using SC for any potting so I'll have tio try it first and see... Thanks... Nutz
Just remember that you can't effectively use tools on the silicon carbide once it sets. It will make short work of just about any tool that you have.
And potting with Arctic Silver??? At dollars per gram that gets rather expensive very fast. By "potting" we are talking about using ounces to pounds of the stuff at a time to encapsulate electronics in a solid block of material.
I've use thermal epoxy made with SiC for years to mount temperature sensors and have potted up lots of circuits (nothing overly high-impedance though). Works very well.
I just did another test to see if material density has any measurable effect. This time I put the powder in a metal cup and compressed it to around 500 PSI in an arbor press (using a nickel as the top anvil). Still no signs of conductivity. The meter that I used this time maxes out at 2 gigohms.
Also tried mixing/wetting it with DD water... still no conductivity.
Another useful tip when mixing/vacuum processing epoxy is to add a few drops of silicone oil. It helps break down the bubbles. Go to your local hobby store and and buy the thinnest viscosity RC race car shock absorber oil. Typical usage would be around 1 drop per up to 100 grams.
Looks like in your testing Texaspyro, that the mix should not be capacitive either.... if you have any data or experience as to SC's capacitive properties in epoxy I'd love to hear about it. I'm not going to be using potting epoxy on threads so the abrasive properties of SC shouldn't be an issue. Thanks for your input - I appreciate you sharing your experience in potting electronics with me and the BLF.... Nutz
Silicon carbide has a dielectric constant of around 10. Epoxy around 3.5 Boron nitride is around 4.5 Air is around 1.0 What this means is that if you fill the gap between two pins on a chip with silicon carbide, it will have around 10 times the capacitance than when the gap is filled with air.
For most applications, it won't be an issue unless you are running at microwave frequencies. I once potted a rather finicky 216 MHz tracking transmitter in the stuff and it made no difference in its operation. It was so sensitive that if you got you hand near the thing, it changed it pulse rate and the frequency shifted.
To get the best thermal conductivity out of a material you want to get the solids packing density up as high as possible. This is usually done by using several sizes of powder (generally past three sizes is not very effective). For instance to get the highest density of oxidizer in a solid rocket propellent there might be 15% 400 micron, 70% 200 micron, and 15% 90 micron particles. It is an art/science working out the optimum particle size distribution. I'm lazy/cheap and use all 200 micron. The performance difference is only a few percent.
A bigger effect is the solids loading in the mix. For potting modules you probably need a pourable mix... not good since epoxy conducts 1/100 the heat of silicon carbide. Better thermal performance happens when you have a very thick, putty like mix. But this can be hard to fill voids, etc with. Even the best diamond based thermal epoxies have about 1/8 the thermal conductivity of pure silicon carbide. (God's law of thermal interface materials: they all suck, most really suck).
Strange as it seems, there's more to life than flashlights!
I've potted electronic systems that were several hundred cubic inches. Then there's the guy that potted up around 15 grand of techno-goodies but forgot to connect an internal cable. Or the people that potted up several hundred rather expensive units that had a fuse on board that tended to blow. Both fiascos used rather un-reworkable potting compounds.
If you're stuck on flashlights, you're using too much epoxy. Try red fuming nitric acid. It dissolves epoxy. Or maybe dichloromethane (aka methylene chloride). Not as effective, but leaves the skin more intact.
And whatever you do... put your hands up and step away from the cyanoacrylate. What's that you say... why can't you put your hands up? Oh yeah, and don't use superglue in the nude... way too embarrassing.